1. Field of the Invention
The present invention relates to a sheet feeder for use with a copying machine, a printing machine, or the like.
2. Prior Art
There are known various sheet feeders for holding a stack of sheets and successively supplying the sheets to copying machines, printing machines, or the like.
One conventional sheet feeder is of the deck type which supports a stack of sheets on a horizontal deck. The sheet feeder is movable between a sheet feeding position in a copying machine, a printing machine, or the like and an exposed position out of the machine, so that the sheet feeder can easily be replenished with new sheets and serviced. This type of sheet feeder is disclosed in Japanese Laid-Open Patent Publication No. 55(1980)-56927 and Japanese Laid-Open Utility Model Publication No. 55(1980)-7065.
Another known sheet feeder, referred to as a double-deck sheet feeder, has upper and lower deck-type sheet tables disposed in a sheet feeding position in a copying machine, a printing machine, or the like, as disclosed in Japanese Patent Publication No. 50(1975)-38968.
Japanese Laid-Open Utility Model Publication No. 52(1977)-36930 shows a cassette-type sheet feeder which includes a sheet cassette storing a stack of sheets. The sheet cassette is loaded in a sheet feeding position in a copying machine, a printing machine, or the like.
Japanese Laid-Open Patent Publication No. 54(1979)-41734 discloses a hybrid sheet feeder which comprises a deck-type sheet table and a cassette-type sheet table.
Generally, copying machines are supplied with sheets which are of different sizes but constant physical properties. The copying machines are equipped with a plurality of vertically arranged sheet cassettes or tables. In operation, one of the sheet cassettes or tables is selected, and sheets are successively fed from the selected sheet cassette or table into the copying machine. The sheets from the sheet cassette or table are delivered through a somewhat complex feed path such as a curved feed path, but do not suffer a feed failure or sheet jam frequently.
Printing machines, on the other hand, are usually supplied with sheets of varying physical qualities, such as thinner and thicker sheets. If the feed path for feeding sheets from the sheet feeder to the printing machine is complex or excessively tortuous, thinner sheets may fail to be fed successfully and thicker sheets may be jammed.
To avoid the above difficulties, ordinary printing machines are equipped with sheet feeders having only one sheet table from which sheets are supplied through a straight sheet path. When sheets of a different size or quality are to be used, however, the printing machine has to be shut down, and the existing sheets have to be replaced with new sheets of a desired size or quality. Because of this interruption or temporary shut-off of the printing machine, the efficiency of the printing machine is lowered by the sheet replacement.
One solution to the above problem is to employ a plurality of movable sheet tables with casters, and to couple one of the sheet tables which stores desired sheets to the printing machine for feeding the sheets. Since the movable sheet tables are independent of the printing machine, it is time-consuming to position any desired sheet table accurately with respect to the printing machine. Accordingly, the printing machine used with these movable sheet tables are still not efficient enough.
It is desirable that a sheet table for use with a printing machine be able to support a large number of sheets of different types, to allow sheet types to be selected and replaced with ease, and also to facilitate maintenance of the stored sheets.
The conventional deck-type sheet feeder disclosed in the first two publications referred to above can hold a large number of sheets, but has only one sheet storage space. When sheets of a different type (size, thickness, or other properties) are to be used, it is necessary to remove the existing sheets from the sheet feeder and then put the desired sheets into the sheet feeder. Special care must be taken to keep the removed sheets in order. Accordingly, a tedious and time-consuming sheet replacing process is needed regardless of the high-speed sheet feeding operation performed by the sheet feeder and demands for automatization of manual operations.
The double-deck sheet feeder shown in the second publication also has disadvantages. The copying or printing machines with which the double-deck sheet feeder is to be used have a limited vertical space available in their sheet inlet region. When the lower sheet table is used, the upper sheet table has to be empty. The sheet tables cannot therefore be automatically selected. The upper sheet table must be taken out manually.
The cassette-type sheet feeder according to the third publication fails to hold a large number of sheets at one time because of the structural limitations and weight of the sheet cassette itself. If a number of sheet cassettes are set in a vertical array, they should be coupled to respective sheet feed mechanisms.
The hybrid sheet feeder disclosed in the fourth publication is only used as an auxiliary sheet feeder since it cannot store a large number of sheets, as with the cassette-type sheet feeder.
Therefore, any of the conventional sheet feeders cannot support a large number of sheets of varying kinds at one time, have limitations on automatization of sheet table selection and on reduction of the space occupied by the sheet table or tables.